Zinc metallurgy



Patented Oct. 29, 1940 UNITED STATES PATENT OFFICE ZINC METALLURGY No Drawing.

Claims.

Various proposals have been advancedin the past for producing zinc by subjecting zinc oxide to a flash reduction and condensing the resulting zinc vapor. In general, these proposals contemplated injecting a finely-divided charge of zinc oxide and carbonaceous reducing agent, with or without flux, into a heated retort wherein the oxide is reduced in suspension by a suitable reducing agent, e. g. carbon monoxide or methane and immediately thereafter recovering zinc from the gaseous products of reduction by various conventional condensation methods. However, those skilled in the art have been unable to translate such proposals into terms of successful commercial practice.

Subsequently, it was found that the flash reduction of zinc oxide :and condensation of the zinc vaporcould be practiced successfully by con-.

4 ducting the gaseous products of reduction on 9 their way from the flash chamber to the condenser, through a suitable reducing and filtering medium, for example, a bed of incandescent coke, such process being described and claimed in United States Letters Patent No. 2,096,779,

l 1 granted October 26, 1937, to George P. Bartholomew and Edward P. Fleming. ,i The present invention provides certain improvements in the art offlash reduction of zinc M oxide by providing a method of preparing or coni ditioning the charge of zinc oxide and carbonaceous reducing agent so as to increase the rate of the reaction with consequent improvement in the recoveries of zinc.- I

In general, the present invention utilizes as the carbonaceous reducing agent in the charge,

the carbonaceous product resulting from the low,

temperature carbonization of bituminous coal, or other active forms of carbon, it having been found that the presence of an active form of carbon in la the charge increases substantially the rate of reduction over that obtainable by the use of less active forms of carbon, such as, for example, metallurgical coke. Thus, for example, the improved process in its, preferred embodiment,

- makes use of bituminous coal as the reducing component in the charge, the mixture of oxide material and finely powdered coal being prepared by passing the same through a suitable preheating apparatus; such as arotary kiln or a i Wedgeor Herreschofi-type of multiple hearth roaster.

. As it passes through the preheater the charge is ventilated with air and heated to a temperature sufllciently high to expel moisture and volatile components from the carbonaceous com- Application November 17, 1938, Serial No. 240,956

slow or gradual heat build-up in' the charge allows a more coking coal to be treated'than does a short overheated kiln.

Or, the charge of coal and zinc oxide may be subjected first to a heating in the presence of limited amounts of air until the coal is rendered non-coking, the charge being then finally comminuted and heated as above to render the carbon active and substantially free from adsorbed gases.

It is preferable practice to mix the coal or petroleum coke with the oxide charge and grind the same together to a suitable fineness, rather than to grind the coal separately. The mixing and grinding offers the advantages of minimizing the hazard of dust explosions, which is always present dangerously when grinding coal itself, and also a more intimate mixture of the coal and oxide charge is obtained by the simultaneous grinding.

If it be desired to do so, however, in any particular case, the coal may be ground before mixing; and if the coal be a coking coal, a suitable preheating thereof in the presence of controlled limited amounts of air before grinding inhibits any tendency to coke, the resul ing treated coal being groundto a sufliciently ne size, and reheated in the presence of limited quantities of air to such a temperature and for such time as will render active the carbon of the coal. This is then mixed with zinc oxide to form the charge.

In operating the flash reduction process with charges including reactive carbon it is found in practice that the elimination of zinc from the charge is definitely higher than when less reactive forms of carbon, such as metallurgical coke, for example, are employed. This is particularly true when the zinc oxide charge contains slag-forming gangue, as. when substantially pure zinc oxide is used, a slowly reacting coke is satisfactory for reducing the zinc oxide.

Thus it was found in practice that operating atia rate of 230 pounds of charge per hour on a 70:30 mix of zinc oxide sinter heavily burdened with slag-forming material and bituminous coal ground together, and preheated as described above prior to charging into the flash reduction column, the vaporization of zinc from the charge in the column was 99.5%; and again using coal, eliminations of zinc in the flash column amounting to 90% and better of the total zinc in the charge are obtained even when the ratio of coal to the above zincoxide sinter is as low as twenty parts of coal to eighty parts of sinter, and the temperature of the charge entering the flash column is only about 300 to 400 C. instead of 600 to 800 C.

The low percentage of coal in the charge is possible because the coal is several times more reactive than is metallurgical coke. The coal and sinter were mixed, then ground together in 'a ball mill, an air classifying system being used to deliver the mixture to storage. The premixed charge then was heated in an externally-fired rotating kiln to a temperature of about 670 C., a slow stream of air being passed through the kiln, which causes combustion of the volatile hydrocarbons which distilled from the coal, the result being a low-temperature carbon formed at a temperature of about 700 C. to 800 0., this temperature increase being due to the combustion proceeding in the kiln; and the charge to be reduced to zinc accordingly was provided with a highly reactive carbon from a cheap coal, with avoidance of grinding coke, which is a costly operation. In view of the fact that the ventilation of the charge in the kiln inhibits coking of the coal and agglomeration of the particles, the resulting carbon is present in the charge in finely divided condition, no further preparation of the charge being necessaryior its introduction into the flash column.

Tests with metallurgical coke: on the same material as above showed a vaporization of zinc of only from about 50 to 70%, but when substantially pure zinc oxide is used in conjunction with metallurgical coke, the vaporization of zinc is substantially above 90%; but using the slaggy By referring to the Bartholomew and Fleming patent mentioned above, it will be seen that the process of flash reduction of zinc oxide is carried out, in general, by bringingthe flash column to zinc reduction temperature and feeding thereto a pre-heated, finely divided charge of zinc oxide and carbonaceous reducing agent, the zinc oxide being reduced as the charge falls to the bottom of the column. The reduction may be expressed by the reactions and preferably the reduction is so controlled that the oxide is reduced practically in toto and the gases accompanying the zinc vapor, as it passes from the flash column to the condenser, are predominantly carbon monoxide.

' Now, here the oxidic material to-be redu d has associated with it substantial amounts of slag-forming gangue, it seems as though that unless sufficient carbon monoxide actually contacts the suspended particles of zinc oxide in the flash column so that the heat consumed in the reduction reaction keeps the particle below'its fusion point, the particles'will attain the temgreater its excess and/or the finer it must be comminuted, and excess carbon and fineness of particle size cause choking of the system and also cause blue powder formation in the condenser.

The presence of .active carbon in the charge avoids these difliculties and apparently effects the reduction of the oxidic material in substantially theupper portion of the flash column, and before any substantial amount of case-hardening of the particles as postulated above may occur.

It may be mentioned in this connection that there are recognizedtwo independent, experimentally established postulates:

1. That elementary carbon (other than diamond and graphite) exists in'two modifications, active and inactive, or alpha and beta.

2. That all primary{ amorphous carbon consists essentially of a stabilized complex of hydrocarbons, adsorbed on a base of activeor alpha carbon.

Active and inactiv carbon differ in important particulars, including 1. Temperature of formation.

2. Chemical activity or susceptibility to oxi dation.

The above active modification is formed whenever carbon is deposited at relatively low temperatures by chemical or thermal decomposition.

of carbon-bearing materials; in general below 500 to. 600 C. v

The inactive modification results from similar decomposition at higher temperatures, in general above 600 to 700 C. The active form 'is' attacked rapidly by oxidizing agents, the inactive In order to I resembling graphite in this particular. The tem perature at which molecular carbon is set free apparently is the controlling factor in deter- .ssf

mining whether it .is of the active or inactive variety. I

, In the ordinary process of distillation of car'- bonaceous materials, at relatively low tempera-v tures active carbon is first formed by the'thermal 1 decomposition of. unstable hydrocarbon's. By virtue of its' specific adsorptive properties, this active carbon which is first formed adsorbs'a furtherquantity of hydrocarbons, and these 1117- drocarbons thereby are stabilized to a remarksorbed on a base of active carbon is the product which has been referred to above as primary carbon, because it is the original product first. occurring in low temperature distillation of carable extent, so that they are retained under conboniferous materials. Many commercial .grades of' animal and vegetable charcoal are of this character, as are such cokes as result from the low temperature distillation of bituminous coals, mineral bitumens, etc.

Anthracite coals may be regarded as native primary carbons since they have been shown to possess an active carbon base saturated with adsorbed hydrocarbons.

Bituminous coals may be regarded similarly, although their hydrocarbon content :is in great excess of the adsorptive capacity of the active carbon base. Such excess may be removed readily by distillation under proper conditions.

Ordinary coke, on the other hand, is heavily charged with inactive carbon resulting from the gas treatment to which it has been subjected, i. e., the decomposition in it of volatile hydrocarbons at high temperatures.

From the above considerations, it will-be apparent that if adsorbed hydrocarbons be removed from primary carbons without alteration of the carbon of the latter, the adsorptive power and also the chemical reactivity is increased, the active modification of carbon being characterized by a high specific adsorptive capacity for gases and rapid attack by oxidizing agents.

activation of carbon, the resulting product possessing the properties of substantially higher gas adsorption and chemical reactivity than had the originalprimary carbon (e. g. coal). One method of activation is to attack the primary carbon by some form of selective chemical action, the most successful of these is differential oxidation in which the adsorption complex is broken down, and the hydrocarbons oxidized or partially decomposed and distilled away from the active carbon.

This action takes place in the present process upon admission of air to the charge during the preheating, so that the carbon in the charge is at least partially activated during the preheating, and therefore is in condition to be highly reactive when brought to reaction temperature in the flash column.

The importance of this property of rapid oxidation ofthe carbon by oxidizing agents in the process of flash reduction of oxidic zinc becomes apparent when it is recalled that the reduction is effected by the reaction of carbon monoxide on zinc oxide, the carbon monoxide being produced by the reduction of carbon dioxide with carbon (CO2+C- 2CO). This reaction normally is relatively slow. Consequently if the carbon is inactive, there is produced insufficient carbon monoxide to contact with the suspended particles of oxidic zinc materials to produce sufliciently rapid reduction thereof before the temperature of the particles reaches that of their environment. If this latter occurs, it seems that the particles will melt or glaze over with slag, thereby preventing further contact of zinc oxide with the carbon monoxide and terminating the reduction.

In connection with the above consideration, it is to be recalled that the velocity of the endothermic reaction CO2+C- 2CO is extremely slow below 1000 C. Consequently, it is necessary to operate the flash column at high temperatures, preferably around 1100 C. to 1200 C. and as the reaction proceeds while the charge is in free suspension in the furnace the charge must reach at least 1000" C. while so suspended in order for the above reaction to proceed with sufiicient velo ity to produce the requisite amount of CO for eiIecting the reduction. I

Consequently, for eflicient operation, it is requisite that the charge be preheated and introduced into the flash column at as high a temperature as is practicable, in order to conserve as much heat as possible. It is recognized also that active carbon when heated to high temperatures passes into the inactive form. Consequently in practice it is necessary to strike a balance between the reactivity of the carbon and the temperature at which the charge enters the flash column, so that the advantages of temperature will more than overcome the degree of loss of reactiveness suffered by thecarbon in event of any overheating. Accordingly in practice the charge is introduced into the flash column at as high a temperature as is commensurate with the activity of the carbon, the endothermic character of the reactions however maintaining the components of the reactants substantially below that of their environment and enabling the active carbon to do its work at such speeds that the reactions are accomplished before the carbon loses its active properties to a deleterious extent from overheating.

Additionally the chemical "activity of the carbon in this connection is not to be confused with the properties of activated carbon to adsorb gases, as the activity of alpha carbon towards oxygen persists at temperatures substantially above those at which the gas adsorptive properties are impairedv or lost.

What is claimed is:

1. The process for producing zinc which comprises charging a finely-divided, freely-falling charge of oxidic zinciferous material and alpha carbon substantially free from adsorbed gases to a flash reduction retort, and collecting the resulting zinc vapors.

2. The process of producing zinc which comprises preparing a finely comminuted charge of oxidic zinc material and solid carbonaceous reducing agent, preheating the said charge while sweeping the same with controlled quantities of air to render the carbon active and effect at least a partial selective oxidation of hydrocarbons adsorbed on the carbon, subjecting the preheated charge to distillation to liberate zinc as vapor therefrom, and condensing the vapor.

3. The process .of producing zinc which comprises preheating a finely comminuted charge of oxidic zinc material and carboniferous material, preheating the said charge sufllciently to effect a low-temperature carbonization of the carboniferous material while ventilating the charge during preheating by admitting thereto controlled quantities of air to prevent coking of the carboniferous material and to produce a carbon of which at least a substantial amount is in active form substantially free from adsorbed gases and subjecting the preheated charge to distillation to liberate the zinc therefrom by causing the said charge to fall freely through a distillation-environment and efiecting the reduction of zinc material whilethe charge is in suspension in the environment.

4. The process of producing zinc which comprises preparing a flnely comminuted charge of oxidic zinc material and carboniferous material, preheating the said charge sufficiently to effect a low temperature carbonization of the carboniferous material .while ventilating the charge during preheating by admitting thereto controlled quantities of air to prevent coking of the carboniferous material and to produce a carbon of which at least a substantial amount is in active form, showering 'the charge immediately from the preheating and while yet hot through a retort heated to reactive temperature of the charge, thereby eflecting distillation of zinc from the charge while the charge is in free suspension in the retort, and collecting the distilled zinc.

5. The process of producing zinc which comprises preparing a finely comminuted charge of oxidic zinc material and carboniferous material, preheating the said charge sufliciently to effect a low temperature carbonization of the carboniferous material to produce active carbon while ventilating the charge. during preheating by admitting thereto controlled quantities of air to prevent coking of the carboniierous material, and showering the charge through a retort heated to reactive temperature of the charge, thereby effecting distillation of zinc from the charge while the charge is in suspended condition in the retort.

6. The process of producing zinc which comprises preparing a finely comminuted charge of oxidic zinc material and reducing agent comprising essentially bituminous coal in quantity suitable to effect reduction of the zinc oxide, subjecting the said charge to a low-temperature distillation to convert the coal in the charge into active carbon in situ in the charge while preventing coking of the coal, and distilling the said charge in a suitable retort to effect reduction of the zinc oxide to metallic zinc.

7. The process as claimed in claim 6, wherein the reducing agent mixed with the charge is petroleum coke.

8. The process/of producing zinc which comprises preparing a finely comminuted charge of oxidic zinc material and reducing agent comprising essentially bituminous coal in quantity suitable to efiect reduction of the zinc oxide, subjecting the said charge to a low-temperature distillatlon to convert the coal in the charge into active carbon in situ in the charge, while preventing coking of the coal by admitting controlled amounts of air to the charge in sufllcient amounts to sweep volatile materials of the coal away from the charge, and distilling the said charge in a suitable retort to effect reduction of the zinc oxide to metallic zinc.

9. The process of producing zincwhich comprises providing a coking coal, reducing the coal to a suitable size, converting the coal into noncoking active carbon by first preheating the coal in the presence of controlled limited amounts of" to a suitable size, mixing oxidic zinc material with the coal, converting the coal into non-coking active 'carbon in the presence of the oxidic zinc materials by first preheating the coal in the presence of controlled limited amounts'of air to efiect a partial oxidation and distillation of the coal,

grinding together the thus-treated coal and the zinc material to produce an intimate, finely comminuted mixture thereof, and again heating the mixture, in the presence of controlled limited amounts of air to temperatures favoring the conversion of the coal into active carbon, and reducing the mixture to zinc in a suitable retort heated to reduction temperatures of the zinc oxide.

JESSE O. BE'ITERTON. MELVILLE F. PERKINS. 

